1. Technical Field
The present invention relates to optical communications, and more particularly to a carrier frequency offset (CFO) estimator with low complexity and high accuracy for coherent optical orthogonal-frequency-division-multiplexing (CO-OFDM) systems.
2. Description of the Related Art
Coherent optical orthogonal-frequency-division-multiplexing (CO-OFDM) is proposed for high-rate transmissions due to its robustness to chromatic dispersion. However, OFDM is very sensitive to carrier frequency offset (CFO) caused by the instability of local oscillators (LO). Since CFO destroys the orthogonality among the subcarriers, CFO within even a small fraction nf the cnhearrier spacing degrades the hit error rate (BER) performance of OFDM receivers severely. Meanwhile, a simple CFO estimator with high accuracy is required in the CO-OFDM real-time system implementation.
In conventional CO-OFDM systems, a radio frequency (RF) tone is added next to the OFDM signal which can be filtered out to estimate the CFO. In other solutions, direct current (DC) components can also be filtered out to estimate the CFO. Both solutions require a small bandwidth band-pass filter which may not be efficient in high speed real-time implementations. Moreover, the additional RF tone increases the system complexity and the DC component may not always be available.